In recent years considerable effort has been devoted to the study of voltage- and current-controlled bistable switching elements fabricated from amorphous or polycrystalline materials, e.g., for use in data storage devices. These elements transition from a high-resistance state to a low-resistance state in times on the order of microseconds to milliseconds when subjected to a threshold value of current or voltage. Some such devices exhibit zero-bias memory retention, meaning that preservation of the low-resistance state does not require the presence of a holding voltage or current. Zero-bias devices may utilize any of a number of materials known to exhibit bistable impedance with dual conductance states, the most important of these being amorphous semiconducting glasses and amorphous metal oxides.
Amorphous semiconducting glasses (most successfully, chalcogenide glasses) have been used to create non-volatile memory devices. These memory switches are amorphous and can be produced on non-crystalline substrates. Moreover, chalcogenide switches are "radiation-hard," making them attractive for particular applications. In the mid-1970s, a 256-bit, non-volatile, electronically alterable memory (so-called "Ovonic memory") was marketed by Energy Conservation Devices as a device to fill the large gap between volatile alterable random access memory (RAM) and non-volatile, non-alterable read-only memory (ROM). However, interest in glass switches has waned considerably since that time due to the need for expensive manufacturing processes and the complicated pulse trains required to store data. Ultimately, memory switch devices were at a distinct disadvantage when pitted against competing technologies such as electronically erasable programmable read-only memories (EEPROMs).
Metal oxide memory structures--also known as oxide switches--are typically metal-oxide-metal sandwiches. Alternatively, coplanar metal-insulator-metal (MIM) structures can be formed. Metal layers are commonly laid down by vacuum evaporation through masks at pressures less than 10.sup.-5 torr. Precision oxidation of the metal is carried out by any of numerous methods including plasma discharge, thermal oxidation, and anodic processes. Switching and negative-resistance behavior in metal oxides has been reported for nickel oxide, aluminum oxide, titanium dioxide, niobium oxide, and zirconium dioxide, among others. However, the switching behavior of such devices is highly dependent upon oxide thickness and electrode deposition methods. Moreover, nearly all metal-oxide switch devices are susceptible to catastrophic failure in the presence of pinhole defects in the insulator.